This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No(s). 2002-167630 filed in JAPAN on Jun. 7, 2002, which is(are) herein incorporated by reference.
1. Field of the Invention
The present invention relates to a novel polymethine compound, an indolenine compound and an intermediate thereof, and near-infrared absorbing materials comprising the polymethine compound. More specifically, it relates to a polymethine compound excellent in absorptive power in the near-infrared region of 750-950 nm but less absorptive in the visible region, hence useful not only as a near-infrared absorbing material for use in laser beam-exploiting plate-making, laser thermal recording media, toners for flash fixing electrophotography but also as a dye for security ink, a dye for near-infrared absorbing filters, a spectral sensitizing dye for electrophotography or silver halide photography, and a dye for optical disk use, among other applications.
2. Description of the Prior Art
Various organic dyes sensitive to rays in the near-infrared region, inclusive of phthalocyanine dyes, polymethine dyes, aluminum-based dyes, diiummonium dyes, and dithiol metal complex dyes, among others, have been proposed in the art and, in particular, phthalocyanine dyes and polymethine dyes, which are excellent in absorbing capacity or absorptive power, have been utilized in various fields.
Generally, phthalocyanine dyes have good light resistance and durability but are low in solubility in various solvents. They absorb light in the visible region as well. Therefore, the range of their application is restricted. Polymethine dyes are low in light stability, hence have limited uses.
Modifications derived from polymethine compounds by introduction of a cyclic structure into part of the methine chain as means for improving the durability of such compounds are known in the art. For example, the following compound A is described in Laid-open Japanese Patent Application (JP Kokai) S63-319191, page 3, specific compound No. 9, and the following compound B in the Journal of Organic Chemistry, 60, 2392, Table 1. 
However, both the compounds A and B show an absorption maximum in the wavelength range of 785-815 nm. Thus, they are insufficient in sensitivity to those small-sized, high-power lasers which are currently under intensive investigation for the practical use thereof and have an emission wavelength range of 820-840 nm. As for the absorptive power in the range of 850-950 nm, which is highly required of near-infrared absorbing materials, they are low as well. The compounds A and B are both low in solubility in solvents and in compatibility with resins. Thus, the binder resins and other ingredients that can be used are limited. A further drawback is that they are discolored upon processing to give films and the like.
A primary object of the present invention is to provide a novel polymethine compound showing intense absorption in the near-infrared region of 750-950 nm, having high solubility in solvents and high compatibility with resins, showing low absorptivity in the visible region, having high transparency and showing a tendency toward discoloration only to a slight extent upon processing to give films or the like.
Another object of the invention is to provide 5-anilino-2,3,3-trimethylindolenine, which is one of the important intermediates for the production of the polymethine compound mentioned above.
A third object of the invention is to provide a near-infrared absorbing material comprising the polymethine compound mentioned above.
As a result of various investigations made to accomplish the above objects, the present inventors found that certain novel polymethine compounds show strong absorption in the near-infrared region (750-950 nm), in particular in the range of 850-950 nm and show only weak absorption in the visible region and, therefore, can be used as near-infrared absorbing materials readily processed for application in various fields. Based on such and other findings, the present invention has now been completed.
In a first aspect, the present invention provides polymethine compounds represented by the general formula (I): 
wherein R1 and R2 each independently represents a hydrogen atom, an alkyl group or an alkoxy group, R3 represents an alkyl group, which may optionally be substituted, L represents an alkylene group necessary for the formation of a cyclic structure, X represents a hydrogen atom, a halogen atom or a substituted amino group, and X represents an acidic residue.
In a second aspect, the invention provides 5-anilino-2,3,3-trimethylindolenine, which is useful as an intermediate for the production of the polymethine compounds according to the first aspect of the invention and has the following formula: 
In a third aspect, the invention provides near-infrared absorbing materials containing any of the above polymethine compounds according to the first aspect of the invention.